Otis Singleton
The discovery of synapses and the subsequent debate over their nature of transmission has been a long and contentious journey. The term synapse was first coined in 1897 by C.S. Sherrington, but it was Du Bois-Reymond's work in 1848 that first sparked the question of whether transmission between nerves and muscles was chemical or electrical. This sparked a long-standing disagreement known as the War of Soups and Sparks, with electrophysiologists championing electrical transmission and pharmacologists advocating for chemical transmission. While the discovery of chemical synapses and their role in neurotransmission was a significant milestone, it is now known that both electrical and chemical synapses coexist in nervous systems, with electrical synapses playing a crucial role in synchronizing electrical activity among neurons.
| Characteristics | Values |
|---|---|
| Date of discovery | N/A |
| First proposed by | Du Bois-Reymond (1848) |
| First named by | C. S. Sherrington (1897) |
| Types | Chemical and electrical |
| Location | Between neurons |
| Function | Transmit information from one neuron to another |
| Chemical synapses | Dependent on the release of neurotransmitter molecules from synaptic vesicles |
| Electrical synapses | Membranes of the two communicating neurons are physically connected by channel proteins forming gap junctions |
| Gap junctions | Contain precisely aligned, paired channels in the membrane of the pre- and postsynaptic neurons |
| Gap junction pores | Large enough to allow molecules such as ATP and second messengers to diffuse intercellularly |
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What You'll Learn
Du Bois-Reymond's early theory
Emil du Bois-Reymond is regarded as the founder of modern neuroscience. In the 1840s, du Bois-Reymond investigated animal electricity, recognising the value in two conflicting theories at the time. The first theory, by physician Luigi Galvani, stated that the action of nerves and muscles was due to the vital powers of "animal electricity". The second, by physicist Alessandro Volta, suggested that muscular contractions were an artifact of electricity generated by the contact of metal with organic tissue. Du Bois-Reymond solved the dispute by demonstrating the electrical nature of nerve signals, creating the discipline of electrophysiology. He was the first to record an action potential.
Du Bois-Reymond's work translated into a repudiation of vitalism, which was the belief that living organisms are fundamentally different from non-living entities because they contain some non-physical element or are governed by different principles. He was not the first to adopt this position, but he was the first to effect a lasting change in the outlook of his colleagues. His work set forth a program of biological reductionism, which demonstrated the electrical nature of nerve signals.
Du Bois-Reymond's investigations of animal electricity were deliberate. He realised that popularizers of science are often the ones who persist in the public mind, even if they are not the originators of the research. Thus, he also made sure to address his audience. In an 1853 letter to his fiancée, he wrote:
> "The time of poetic production in European nations appears to have passed, and talent, which might otherwise have achieved something there, throws itself into oratory and journalism, and later dabbles in politics."
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The War of Soups and Sparks
The "War of Soups and Sparks" refers to a long-standing disagreement between proponents of electrical and chemical synaptic transmission. The "sparks", championed by electrophysiologists, refer to electrical transmission, while the "soups", championed by pharmacologists, refer to chemical transmission.
The controversy surrounding the nature of synaptic transmission dates back to the pioneering work of Du Bois-Reymond in 1848. Du Bois-Reymond was the first to record an action potential and initially proposed that transmission at the junction between nerves and muscles could be either chemical or electrical. However, thirty years later, he changed his mind and favoured a chemical mediator at the motor end plate, such as lactic acid or ammonium.
Santiago Ramón y Cajal and Charles Sherrington, considered the fathers of modern neuroscience, established that networks of neurons communicate via functional specializations called "synapses". Sherrington, who coined the term "synapse" in 1897, defined synapses as having two important characteristics: a valvular effect and a synaptic delay. Both of these characteristics spoke in favour of chemical transmission.
Despite Sherrington's contribution, the debate over the nature of synaptic transmission continued. Proponents of electrical transmission, or "sparks", argued for the presence of electrical synapses in the mammalian brain and nervous systems. They highlighted the role of gap junctions, which are intercellular specializations that physically connect the membranes of pre- and postsynaptic neurons. Electrical transmission occurs through the passive flow of current and the diffusion of molecules through gap junction pores, allowing for extremely fast and synchronized communication between neurons.
On the other hand, proponents of chemical transmission, or "soups", emphasized the role of neurotransmitters and the release of active substances from presynaptic neurons onto receptive elements of postsynaptic cells. Chemical transmission involves the fusion of vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. These neurotransmitters then diffuse across the cleft and bind to receptor proteins on the postsynaptic membrane, resulting in localized depolarization or hyperpolarization. While chemical transmission has a millisecond delay, it allows for the transmission of complex signals and the integration of information.
Ultimately, it was concluded that both electrical and chemical transmissions coexist in all nervous systems and brain structures. However, the debate between "soups" and "sparks" played a crucial role in advancing our understanding of synaptic transmission and the complex mechanisms underlying neuronal communication.
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The role of glial cells
The history of the discovery of electrical and chemical synapses is a fascinating one. The "War of Soups and Sparks" is a humorous name for the long-standing disagreement between proponents of electrical and chemical synaptic transmission. In 1848, Du Bois-Reymond, an early proponent of electrical transmission, recorded an action potential for the first time. However, he later changed his stance, concluding that the synaptic mediator at the motor end plate should be a chemical substance, either lactic acid or ammonium. Santiago Ramón y Cajal and Charles Sherrington, considered the fathers of modern neuroscience, established that networks of neurons communicate via functional connections called "synapses".
Now, onto the role of glial cells. Glial cells are a type of cell that provides physical and chemical support to neurons and maintain their environment. They are located in the central nervous system and peripheral nervous system and are sometimes referred to as the "glue" of the nervous system. Glial cells have a supportive role, ensuring that neurons can perform their functions effectively.
Glial cells are closely associated with synapses and play a crucial role in synapse development and function. They can help establish, maintain, and reconstitute synapses. Evidence suggests that neurons require support from glial cells to form and maintain connections, particularly in the case of chemical synapses. Glial cells possess most of the neurons' neurotransmitter receptors and ion channels, enabling them to directly influence nervous function and modulate synaptic transmission.
Additionally, glial cells can prevent the formation of ectopic sprouts and contribute to establishing the stereotypical morphology of neurons. For example, in Drosophila, the elimination of neuroglian, an adhesion molecule, caused ectopic axonal sprouting and dendrite deformation in a specific sensory neuron. When neuroglian was re-expressed in both neurons and associated glial cells, the phenotype was rescued, indicating the importance of glial cells in maintaining neuronal structure.
Furthermore, glial cells are involved in regulating metabolism in the brain. Astrocytes, a type of glial cell, store sugar (glucose) from the blood and provide it as fuel for neurons. Astrocyte dysfunction has been linked to neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Glial cells also play a role in synchronizing the activity of axons, which are long, thread-like parts of neurons that conduct electricity to transmit messages between cells.
In conclusion, glial cells have a crucial role in supporting neurons and maintaining their environment. They are involved in synapse development, neurotransmission, neuronal morphology, metabolism, and axonal activity. Their diverse functions contribute to the overall effectiveness of the nervous system.
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The evolutionary origins of chemical synapses
Chemical synapses are a type of contact point between neurons, enabling them to communicate with each other and with non-neuronal cells. This communication occurs through the release of active substances from the presynaptic neuron, which induces a wave of electrochemical response in the postsynaptic cell. This process, known as synaptic transmission, is fundamental to neurobiology and facilitates the exchange of metabolites, messengers, and signals between cells.
Recent genomic and molecular analyses have revealed that the key ingredients of synapses, including proteins, are not neuron-specific. These analyses suggest that the majority of molecular scaffolds and ingredients of synapses emerged independently before the rise of neurons. This finding indicates that the evolutionary journey of synapses involved dramatic changes and remarkable conservation of certain constituents from unicellular eukaryotes to vertebrates.
Furthermore, the evolutionary origin of chemical synapses may be linked to the emergence of multicellular living forms. Evidence suggests that key components of chemical synapses arose independently from neurons in different functional and biological contexts before multicellular life. This implies that distinct synaptic constituents were co-opted from ancestral forms, leading to the rise of chemical synapses and neurotransmission in early metazoans.
Additionally, the recent sequencing of genomes from non-bilaterian metazoans and their closest relatives has provided insights into the presence of proto-synaptic genes when the first metazoans appeared. This suggests that parts of the synaptic signaling machinery may have been co-opted from ancestral roles, potentially involving electrical and/or chemical signaling using proto-synaptic proteins.
In conclusion, while the exact evolutionary origins of chemical synapses remain obscure, recent findings have shed light on the complex evolutionary journey of these specialized contacts. The emergence of chemical synapses has played a pivotal role in advancing cell-to-cell signaling and the evolution of multicellular organization.
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The coexistence of electrical and chemical synapses
The existence of electrical and chemical synapses has long been a subject of debate. The two modalities of synaptic transmission do not function independently, and mounting evidence indicates that they closely interact during development and in the adult brain.
Electrical and chemical synapses are known to coexist in most organisms and brain structures, but the details of their properties and distribution are still emerging. For instance, electrical synapses were thought to be more abundant in invertebrates and cold-blooded vertebrates and less prevalent in mammals. However, recent data indicates a widespread distribution of electrical synapses in the mammalian brain.
The neuronal membranes involved in chemical synaptic transmission are highly specialized, with each subdomain having a very different structure. The presynaptic subdomain is specialized for the release of neurotransmitters, while the postsynaptic subdomain is equipped with receptor proteins required for efficient neurotransmission. In contrast, electrical synapses are neuronal gap junctions forming intercellular channels through which ions and small metabolites can pass. These channels are formed by the apposition of connexin (Cx) proteins in vertebrates and innexin (inx) proteins in invertebrates.
The two modalities of synaptic transmission have distinct characteristics. Chemical synapses depend on the release of neurotransmitter molecules from synaptic vesicles to pass on their signal, resulting in a delay between the axon potential reaching the presynaptic terminal and the neurotransmitter release. On the other hand, electrical synapses involve the direct transfer of current from one cell to the next through gap junctions, with no delay.
The interaction between electrical and chemical synapses is complex and can have profound functional consequences. For example, nearby glutamatergic synapses can regulate electrical transmission via NMDAR or mGLUR activation, leading to simultaneous enhancement of electrical and glutamatergic synaptic transmission. In addition, electrical and chemical synapses can mutually coregulate each other's formation.
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Frequently asked questions
Synapses are the contact points between neurons, where they communicate with each other or with non-neuronal cells.
The term "synapse" was first used in 1897 by C. S. Sherrington, but the existence of chemical synapses was not confirmed until much later. The first evidence in support of electrical synaptic transmission was provided by Michael V.L. Bennett, David Robertson, and Edwin Furshpan, who identified cellular structures called gap junctions.
The first evidence for electrical synaptic transmission was provided in the studies by Michael V.L. Bennett, David Robertson, and Edwin Furshpan. They identified cellular structures called gap junctions, which are now known to be a key feature of electrical synapses.
Chemical synapses rely on the release of neurotransmitter molecules from synaptic vesicles, while electrical synapses have physically connected pre- and postsynaptic membranes with gap junctions that allow for the direct passage of current and the diffusion of various substances.
